FORT WORTH, Texas — A knock on the door of a modest wood-frame house on Nov. 3 led an undercover Fort Worth police officer to two Texas Christian University football players.

At that moment, TCU’s highly respected football team — less than a year removed from an undefeated season and a Rose Bowl victory — was about to be drawn into a drug investigation that would threaten the program’s image and rock the campus of 9,100 students.

Two days before the football team played in Laramie, Wyo., undercover officer J.C. Williams showed up on Brady Avenue, about a mile south of campus, at 9:35 p.m. with cash in hand. A confidential police informant who knew the residents there told them a friend would drop by for some marijuana, according to arrest warrant affidavits.

Williams was welcomed into the house and met Ty and D.J., according to the documents. The officer later confirmed they were sophomore offensive lineman Ty Horn and junior defensive tackle D.J. Yendrey.

Waiting for the half-ounce of marijuana, Williams said to Horn: “$220 … is that cool?” according to police.

“Horn said, ‘Ya,’ and I handed Horn $220,” according to Williams’ account.

That alleged drug buy was part of a six-month investigation that led to the arrest last week of Horn, Yendrey, junior linebacker Tanner Brock and redshirt junior safety Devin Johnson. The four players were among 15 students arrested — all of whom were then expelled by the school.

Six others also were arrested on similar charges based on evidence related to suspected drug dealing on or near the TCU campus. Another suspect turned himself in Friday.

Several other drug arrests connected to TCU two years earlier barely caused any ripples. But none of the students apprehended then were prominent athletes on a team that flirted with a national championship in 2010.

Until last week, TCU had been basking in the attention that came from its decade-long football resurgence and a move from the Mountain West Conference to the higher profile Big 12. Even TCU’s unranked basketball team attracted sports fans’ attention by beating 11th-ranked University of Nevada-Las Vegas on Tuesday night.

It was the last good news at TCU last week. Hours after the victory, police were on campus and at student residences off campus, making the arrests.